Resonance lamps for very low voltages



y 0, 1958 M. LAPQRTE A 2,835,840

RESONANCE LAMPS FOR VERY LOW VOLTAGES Filed Dec. 7, 1956 INVENTOR MARCELLAPORTE lfwzm ATTORN E) 5 ats RESQNAllCE LAMPS FOR VERY LGW VOLTAGESltlarcel Laporte, Paris, France, assignor to Centre National de laRecherche Scientifique, Paris, France, a corporation or France It isknown to use, for lighting purposes, cylindrical tubes which aregenerally straight, containing mercury vapour and a rare gas under apressure of a few millimeters of mercury, the gas being especially argonor a mixture of rare gases, for example argon-krypton.

An electrode is mounted in each extremity of the tube; these twoelectrodes are identical, each of them being connected to one of theextremities of a filament which carries a coating of a material having ahigh thermo-electronic emission. This emission is produced by thepassage of a heating current through the filament; it is only producedwith the object of facilitating the starting of the discharge, and it isstopped by the automatic interruption of the heating current as soon asthe ignition of the tube has been obtained. From that instant, the tubeoperates in autonomous discharge, that is to say without any supply ofexternal energy other than that which is supplied in the circuit of thedischarge itself.

it is known that, in accordance with this. type of dis charge, thecharacteristic v:f(i) of the tube is negative, which means that anincrease in the intensity of the current produces a reduction in thevoltage of operation.

The light emitted by these tubes comprises on the one hand theradiations emitted by the mercury vapour in the visible spectrum, and onthe other hand, the radiations emitted by the fluorescence of thesubstance deposited a thin layer on the internal wall of the tube. Thisfluorescence, with a continuous spectral base, is excited by theultra-violet radiations emitted by the mercury vapour, and especially bythe radiation of resonance of the mercury having a wave-length \=2.537Angstrom units.

An economic production of light is only obtainable with these tubes ifthey have a minimum length of several decimetres which make it necessaryto have a maintenance supply voltage greater than 100 volts. By reasonor" the length of the tubes, the current supply leads must be separatedin order to be joined to the electrodes; in order to conceal theseleads, it has been the practice to place them inside a casing arrangedparallel to the tube over its entire length.

The ignition of these tubes necessitates in addition an auxiliary devicefor pre-heating the filaments and for the creation of a momentary surgeof voltage; finally, their supply circuit must necessarily include astabilising impedance by reason of the fact that they have a negativecharacteristic. A power which is of the order of 20 to 25% of that whichis consumed in the lamp is expended as a pure loss in the stabilisationdevice.

The lead-containing casing, the auxiliary ignition and stabilisingdevices (the purchase price of which is several times that of the tube)cannot be easily moved by reason of their weight and their bulk, and theresult is that fluorescent tubes are generally fixed in permanence towalls or to ceilings.

The present invention provides a remedy for the various lrawbacks whichhave been referred to above; it re lates to lighting lamps which will beknown as resonance lamps inthe text which follows, by reason of the factthat there is employed in them the propagation from place to place andin all directions, of the radiation of resonance of the mercury vapour;the ordinary commercial tubes, as at present known, will be termedfluorescent tubes.

In the drawings:

Fig. l is a schematic sectional view of the invention.

Fig. 2 is a circuit showing an alternate form of the invention.

Fig. 3 is a graph showing the relation between current and voltage.

The resonance lamps comprise a glass chamber, the internal wall of whichis coated with a fluorescent powder; their shape is no longernecessarily tubular; it may be of considerable variety-bulbs, balloons,plates, etc., or all shapes which are compatible with a mechanicalstrength sufficient to withstand the force of atmospheric pressure.These chambers will contain mercury vapour and argon (or as the case maybe, a mixture of argon and krypton), at a pressure which will besuitably chosen to give the optimum working of one type of lamp; thispressure will in general be lower than that employed in fluorescenttubes and may be for example in the vicinity of one millimeter ofmercury only.

Inside the glass chamber, there will be arranged in proximity to eachother, that is to say at a distance which will not exceed a fewcentimeters, a cathode having a high thermo-electronic emission,preferably indirectlyheated and of the type with a sintered cathode, andan anode formed preferably by one or a number of grids arranged so as tocollect to the maximum extent the electrons emitted by the cathode. Itis essential to observe that the cathode of a resonance lamp must beheated for the entire duration of working of the lamp and not only atthe moment of ignition as is the case with ordinary fluorescent tubes.

In these conditions, as long as the potential established between theelectrodes is less than 4.6 volts, which po tential corresponds to theminimum excitation of the mercury at the first level of resonance, noemission of light can be observed and only a very smallthermo-electronic current is set up in the lamp. When the voltage beginsto exceed 4.6 volts, at certain number of atoms of mercury are excitedby electronic shocks at the level of resonance, and the return of theseatoms to their normal state is accompanied by the emission of theradiation x=2.537 Angstrom units, which excites the luminousfluorescence of the powder.

As the potential difference between the electrodes is progressivelyincreased, there is observed, at a certain critical voltage V a sharpincrease in the intensity of the current passing through the lamp and acorresponding sharp increase in the light emitted, a phenomenon whichgives the impression of an abrupt ignition of the lamp.

This increase in current is due, to a major extent, to the fact that atthe voltage V which will be called the ignition voltage, the electronicshocks begin to cause the appearance of positive ions; these ions areattracted towards the cathode and neutralise the space charge due to theelectrons; the thermo-electronic emission, which was previously verysmall, then increases sharply with a corresponding increase in thenumber of electronic shocks capable of exciting the mercury vapour.

Experience has shown that the voltage V, of ignition ot' the cold lampis in the vicinity of 12 volts, which is slightly greater than theexcitation potentials of argon at its resonance level or at the adjacentmeta-stable levels. The explanation of this ignition is to be found inthe fact that the excited atoms of argon can ionise atoms of mercury byshocks of a secondary nature; the positive ions of mercury formed bythis means, reach the cathode and more or less completely neutralise theelectronic space charge which tends to hinder the emission.

If, as a result of its previous operation, the lamp is still hot enough,the pressure of the mercury vapour may be sufliciently high for theprobability of collision between the electrons and the mercury atoms notto be negligible; a recently extinguished lamp can be re-ignited at avolt age only very little greater than 10.4 volts, which corresponds tothe potential of direct ionisation of the mercury vapour.

In the case of a lamp which is still hot and with a cathode having avery high electronic emission, it is even possible to obtain a're-ignition at a voltage only very little greater than 5.4 volts, whichis the voltage corresponding to the excitation of an atom of mercury ata meta-stable level; it is only necessary that such an excited atomshould be subjected to a new collision with an energy of 5electron-volts, to be ionised; if such cumulative collisions aresufficiently numerous, the space charge surrounding the cathode will beneutralised and the re-ignition of the lamp will thus be obtained at avoltage only a little greater than 5.4 volts.

It is important to note that if the space charge is completelyneutralised, the thermo-electronic emission from the cathode cannotexceed a limiting value which is determined, in accordance withRichardsons law, by its surface area and its temperature. In addition,the cur rent which passes through the lamp is the sum of thethermo-electronic current and the ionisation current; the latter cannothave a high value since, at the low voltages employed, each electron inits passage between the cathode and the anode can only ionise a verysmall number of atoms; it is thus necessary to ensure that the currentwhich passes through a resonance lamp cannot exceed a limiting valuewhich is only a little greater than the saturation current which thecathode would supply, in a vacuum, at the same temperature. Thisprovision has been confirmed experimentally in the curve of thecharacteristics, and by way of example, there has been shown in Fig. 3one of the characteristics obtained by experiment.

The-necessity of use of an auxiliary rare gas, and the reason for afavourable choice of its pressure can be explained in the following way:when the resonance lamp is at the ambient temperature, the pressure ofthe mercury vapour in it is very small, and the free mean path of theelectrons, in the mercury vapour, is greater than the distance from thecathode to the anode; under these conditions, in the'absence of raregas, the probability that the electrons emitted by the cathode canionise the mercury vapour is too small for ignition, which necessitatesthe arrival on the cathode of a sufiicient number of positive ions, totake place. If the argon is introduced into the lamp at a pressure ofthe order of one millimeter, much greater than that of the mercuryVapour, the probability of excitation of the argon will no longer benegligible, as the ionisation of the mercury may be effected bycollisions of a secondary nature with the excited argon atoms, the freepath of which in the mercury vapour is much smaller than that of theelectrons and which, in addition, are not controlled by the electricfield.

Experience has shown that in order to obtain operation of a resonancelamp with a voltage in the vicinity of 12 volts, there exists an optimumvalue of the pressure for every anode-cathode distance; for example, foran anodecathode distance of 15 millimeters, the optimum pressure is inthe vicinity of one millimeter of mercury.

The necessity for limitation of the pressure of the argon would appearto result from the losses of energy to which the electrons are subjectedin their elastic collisions with the atoms of argon; if the pressure istoo high and if, accordingly, these collisions are too numerous in thepath from the cathode to the anode, the electric fieid will not be ableto give them the energy necessary for the direct or indirect ionisationof the mercury vapour.

Experience has also shown that the characteristic of a resonance lamp isvery different from that of ordinary fluorescent tubes. This result isnot surprising since their methods of operation are totally diiierent:fluorescent tubes operate with an autonomous discharge, whilst resonancelamps operate with a semi-autonomous discharge; the current which passesthrough them falls in fact to zero immediately under the low voltagesemployed, if the heating of the cathode is stopped, the energy of thisheating not being supplied in the discharge circuit.

The form of the characteristic v=f(i) of a resonance lamp depends on thegeometric parameters which define the relative arrangement of theelectrodes and, for any given lamp, on a number of other factors:

(1) On the intensity of the thermo-electronic current which the cathodeis capable of supplying, that is to say on the nature of the saidcathode, on its surface area and on the temperature to which it isbrought;

(2) On the Working temperature of the lamp which determines the pressureof the mercury vapour; this pressure is a function of the power expendedin the lamp and on the conditions under which it is cooled;

(3) On the pressure of the rare gas.

Tests in connection with the influence of these various factors haveshown that the characteristics which are first of all negative in thecase of low currents become clearly positive at currents above a certainvalue.

The very important result of this study has been to establish that it ispossible to define, by the choice of the pressure of the argon and bythe choice of the heating voltage for the cathode, a field of currentintensities in which the characteristic is very clearly positive and inwhich, in consequence, stable operation can be obtained either without astabilising member or, if too great variations of the supply voltage areto be feared, with a stabilising member of very small bulk whichconsumes only a very small fraction of the total energy of the supply.For example, for a type of lamp having a sintered cathode, the heatingof which absorbs 13 Watts, in which the cathode-anode distance is 15millimeters and the pressure of the argon is 1.26 mm. of mercury, thecurrent intensity only varies from 2.2 to 2.6 amperes when the voltagebetween the electrodes varies from 8.7 to 104 volts: this lamp beingignited from the cold state at about 10 volts, it will be seen that itsoperation does not require any stabilising member if a generator isemployed, the electro-motive force of which is only slightly greaterthan 10 volts.

In a resonance lamp, the emission of the radiation of resonance is firstof all produced in the space comprised between the cathode and theanode, but it is known that these resonance radiations can be absorbedby adjacent atoms and then re-emitted by them in any directions. Theradiation of resonance is thus diffused from place to place, and this inall directions, and it can thus reach the walls of the chamber andilluminate the fluorescent deposit on the walls, whatever the form ofthese walls may be. Objects of any shape, placed inside the lamp andcovered with a suitable fluorescent deposit can also be illuminated;they can be seen through parts of the walls of the lamp which are notthemselves provided with a fluorescent deposit. It is clear that thispossibility can be used for publicity purposes and for new decorativeelfects.

The anode and the cathode of a resonance lamp being arranged at a shortdistance from each other, the input and output passages for the currentmay be effected in the same base; on the other hand, by suitably chosingits resistance, the heating filament may be connected in parallelbetween the cathode and the anode; the complete supply of the lamp canthen be efected by means of two wires very close together: resonancelamps will thus have the advantage over tubular lamps, which up to nowwas reserved for incandescent lamps, of constituting portable sources oflight whilst at the same time they retain the luminous qualitiespeculiar to fluorescent sources.

By way of indication and without any sense of limitation, there will nowbe described a number of forms of embodiment of the invention, referencebeing made to the diagrammatic drawings shown in the accompanying Figs.1 and 2 and which concern respectively a lamp 'ng the shape of a bulb ascurrently employed, and an. alternative form of this device whichcomprises two anodes and two cathodes, with the object of reducing theflickering of the flow of light when the lamp is supplied withalternating current.

Referring now to Fig. l, the device comprises a glass bulb 1 of anydesired shape, coated internally with a fluorescent substance 2 andprovided with a pipe 3 which is necessary for filling the bulb with raregas and mercury vapour, after having produced a very high vacuum in thebulb. The device also comprises an oxide cathode 4, preferably of thesintered type with indirect heating, provided by means of a heatingfilament 5. The anode 6 is preferably in the form of a grid.

These members are connected to the conducting wires 7 and 8 as shown inFig. l, and finally these conductors are connected to the supplycircuit, which may comprise an inductance coil or regulating resistance9 which may be dispensed with if too great variations in the supplyvoltage not to be feared. The supply circuit also comprises alow-tension source of alternating or directcurrent voltage of about 12volts, applied across the terminals Hi and ii.

When the lamp is supplied at a constant voltage, for example by means ofan accumulator battery, the light radiation is constant. In the case inwhich the lamp is supplied from an alternating voltage source, forexample at a frequency of 50 cycles per second, there is observed in thecase of a system of connections as shown in Fig. l, a slight flickeringwhich results from the fact that the charge only passes during afraction of one of the halfwaves of voltage.

This defect is very small and is eliminated in practice if the lightingis obtained by means of a double circuit employing two lamps, the anodeof one being connected to the pole of the secondary of the transformerto which is connected the cathode of the other lamp. The same result canbe obtained with a single bulb con-- taining two anodes and twocathodes, for example in accordance with the circuit showndiagrammatically in Fig. 2.

in this figure, the two filaments l2. and 13 are connected in parallel;during one half-wave, the discharge passes from the anode 14 to thecathode 15 which is heated by the filament 12, and in the nexthalf-wave, from the anode 16 to the cathode 17, which is heated by thefilament 13; the lamp then operates at a frequency which is twice thatof the supply system.

It will be observed that in this diagram the electrodes 14 and 15 areplaced far apart from each other; the same condition holds for theelectrodes 16 and 17, the anodes M and 16 respectively protecting thecathodes 15 and 1'7. In addition, the ignition is controlled by theswitch 13 connected in the circuit of the secondary of the trans former19, which is not obviously in any way combined with the lamp.

In all these devices, the anode may comprise a number of grids suitablyspaced apart in order to ensure an almost complete interception of theelectrons.

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It should be noted that the ignition of the discharge of a resonancelamp is only produced when, on the one hand, the cathode is brought upto a temperature which makes its electronic emission sufficientlyintense and when, on the other hand, the pressure of the mercury vapourhas reached a sufiicient value by reason of the heating-up of thechamber. For this reason, a certain time elapses between the instant atwhich the current is applied and the instant corresponding to theproduction of the normal lighting at full intensity, the duration ofthis delay being reduced directly with the heat capacity of the cathode.It has proved a simple matter to choose this capacity in such mannerthat the delay does not exceed a few seconds. In addition, during thistime, and almost instantaneously with the closure of the currentcircuit, there is produced a progressively increasing lighting effectdue to the emission of light of thermal origin from the heating filamentof the cathode, and then due to that of the body of the cathode, astheir temperature rises. There is thus obtained in a very agreeablemannor, a switching on with progressive lighting effect which startsalmost at the same time as the switch is closed and which reaches itsfull effectiveness in a few seconds. The reverse phenomenon of aprogressively decreasing light is obtained when the lamp isextinguished.

By reason of the fact that the cathode is heated to a fairly hightemperature in a gaseous atmosphere at low pressure, it is preferable toprevent as far as possible the formation of an opaque deposit on thewalls of the lamp, due to vaporisation of the metallic support of thecathode. T 0 this end, the non-emitting parts of the cathode may beenclosed by a sleeve, for example of quartz, which will have in additionthe advantage of reducing the power for heating.

What I claim is:

1. In a resonance lamp for producing fluorescent illumination andadapted to operate from both alternating current and direct currentsources at potentials less than 15 volts, the combination including anexternal envelope at least a portion of which is transparent,fluorescent material on the internal wall of said envelope, saidmaterial having a continuous spectral base, mercury vapour and at leastone of the rare gases within said envelope, the pressure thereof beingabout one millimeter of mercury, at least one thermionic emissionsintered cathode in said envelope, indirect heating means for saidcathode operable during the entire operating period of said lamp, and atleast one grid-like anode in said envelope located adjacent said cathodeat a distance of not more than a few centimeters therefrom, saidfluorescent material being excited by the ultra-violet radiation andparticularly the radiation of resonance of mercury having a wave lengthof 2.537 Angstrom units, the ignition voltage of the cold lamp beingabout 12 volts.

2. A resonance lamp as in claim 1 including a stabilizing impedance tocompensate for potential variations in the supply to the lamp, and abase housing said impedance and leads to said cathode and anode.

References Cited in the file of this patent UNITED STATES PATENTS2,030,805 Wiegand Feb. 11, 1936 2,182,732 Meyer et al. Dec. 5, 19392,222,668 Knoll Nov. 26, 1940 2,409,771 Lowry et a1. Oct. 22, 1946

